The present invention relates to a method and to an apparatus for controlling slag in an arc furnace. The present invention has particular applicability to controlling the nature and quality of the slag during the process for the preparation of metal alloys to improve reliability, consistency as well as the ultimate properties of the manufactured alloy.
Methods for processing steel in an electric arc furnace involve melting mostly scrap metal as the charge to form a liquid bath of the metal. Conventionally, the scrap is added to the furnace at the beginning of the process cycle and melted down by the passage of electric current through the furnace""s graphite electrodes which causes an electrical arc between the electrodes and the metal. A layer of molten, non-metallic material known as slag is present over the upper surface of the molten steel formed from a mixture of the impurities separated from the metal during the refining operation. The slag is formed on the upper surface of the charge during the initial melt-down and remains on the upper surface during the entire process cycle until tapping of the heat.
In conventional steel making, the slag is foamed by the adjustment of furnace parameters and the addition of materials, e.g. liquid or gaseous chemicals, which are added during the refining process. The resulting layer of foamed slag over the upper surface of the molten steel serves the useful purposes of regulating the percentage content of alloy additives in the steel, isolating the molten steel from ambient oxygen and certain devices in the vessel containing the molten steel, and thermally insulating the steel to facilitate maintaining the steel in a molten state until poured into a desired mold.
Conventionally, the slag is foamed by a furnace operator upon visual observations of the slag itself or upon some other process parameters. For example, U.S. Pat. No. 5,827,474 to Usher et al. discloses an apparatus and method for measuring the depth of slag and molten metal in an electric arc furnace. The method includes moving a voltmeter probe through the molten metal, the interface between the molten metal and the slag foam. The change in the voltage signal between the strata, marks the position of the boundary between the molten metal and the slag foam.
Initially, visual inspection of the slag is sufficient to determine the quality of the foam. Over the process time, however, the dynamic process of steel making requires the constant adjustment of variables such as the addition of carbon or oxygen to affect the foam. When furnace operators are adjusting parameters, such as the injecting of carbon, their reaction time is relatively slow in relation to the dynamics of the foaming and alloying. To overcome, in part, some of the deficiencies of human error in foaming the slag, several models have been developed for the steelmaking process. For example, WIPO Patent Application No. WO 99/23264 to Welker discloses a method and a device for controlling the formation of foamed slag in an arc furnace employing a model to predict foaming. The reference discloses the use of a neural net and a foamed slag model to predict the correct amount of carbon to add to the furnace in order to ensure that at least partial envelopment of the arc in the slag foam is achieved. German Laid Open Patent Application (Offenlegungsschrift) DE 198 01 295 to Sesselmann discloses an arc furnace in which a neural net is used to control the position of the arc electrode, the supply of electrical energy and the amount of carbon added to the furnace. The height of the slag foam is measured using sonar.
Additional controls have been made to the furnace in the steel making process. For example, U.S. Pat. No. 5,714,113 to Gitman et al. discloses an apparatus for electric steel making, which comprises a burner/injector for introducing oxygen into the steel melt.
Sensors have also been added to the furnace, as disclosed in U.S. Pat. No. 5,050,185 to Bourge et al. which discloses a process and apparatus for measuring the instability of an arc in an electrical arc furnace for processing liquid metal. This patent discloses that arc stability may be measured by taking the derivative of the signal corresponding to the arc current, splitting the derivative signal into high and low frequency components, amplifying these separate components, and producing therefrom a signal relating to the quotient of the high and low frequency components of the derivative signal. This value represents the stability of the arc and can be used by operators to estimate the amount of carbon and oxygen to be injected into the furnace.
A method for on-line monitoring and control of the performance of an electric arc furnace can be found in U.S. Pat. No. 5,099,438 to Gulden, Jr. et al. The method of Gulden, Jr. et al. includes collecting signals relating to various furnace parameters including electrical, mechanical and physical parameters of the furnace. Such parameters include pulse rates from watt/var, current and potential transducers, transformer tap positions, arc length settings, hydraulic variables, positions of mechanical furnace components and scrap charge weights. These data are processed using programmable logic controllers (PLCs) and microcomputers.
U.S. Pat. No. 4,742,528 to Stenzel discloses a compensating device for use in a vacuum arc furnace. The compensating device compensates for the weight of a power line in a vacuum arc furnace, where accurate weights of the arc electrode are to be determined. The method uses a computer to control the intensity of current supplied to sensor electrodes.
Russian Patent RU 2086657 to Dolgonosova, et al. teaches a device for reduction of metal oxides comprising a loading device for feeding metals into a blast furnace. The device measures gas pressure fluctuations, and uses these signals via a computer to control the feed of oxygen gas into the furnace.
The aforementioned references, however, do not adequately correlate or control the various dynamic aspects of the arc furnace or the dynamic properties of the molten steel or slag to improve the nature or quality of the foamed slag in the manufacture of steel. Accordingly, a continuing need exists for dynamically improving the quality and nature of the foamed slag during the steel making process thereby improving consistency, reliability and throughput for the finished product.
An advantage of the present invention is a method of dynamically and more precisely controlling the foaming of slag in an arc furnace.
Additional advantages and other features of the invention will be set forth in the description which follows and in part will be apparent to those having ordinary skill in the art upon examination of the following or may be learned from the practice of the present invention. Advantages of the present invention may be realized and particularly pointed out in the appended claims.
According to the present invention, the foregoing and other advantages are achieved in part by a method of dynamically controlling the foaming of slag in an arc furnace in the formation of an alloy, such as steel. The method comprises obtaining one or more signal(s) related to variables representing the nature or quality of the slag; feeding the one or more signal(s) into a controller comprising a logic program to generate an output signal representative of the quality or nature of the slag; and adding one or more materials to the electric arc furnace through a valve that has a flow rate that can be variably adjusted to dynamically control the quality or nature of the slag by continuously adjusting the addition of the one or more material in response to the output signal generated by the controller. The method of the present invention thereby dynamically and more precisely controls the quality and nature of the slag, its foaming and the ultimate finished product.
Embodiments of the present invention include employing signals relating to arc stability, slag foam viscosity and temperature as indicators of slag foam quality and continuously controlling the amount of materials to be injected into the furnace to affect the nature and quality of the slag, as by a variable flow rate valve. Further embodiments of the invention employ oxygen, carbon, magnesium oxides, calcium oxides and lime as inputs for controlling the nature or quality of slag foam. The invention also contemplates automatic and manual controls as means for controlling the introduction of inputs to control of the slag foam.
A further aspect of the present invention is a method of manufacturing a metal alloy by forming a molten metal liquid having a slag thereover in an electric arc furnace and dynamically adjusting the foaming of the slag during the process by injecting material, e.g. carbon, through a valve capable of continuously and variable adjusting the flow rate, e.g. from about 25 pound/minute (lbs/min) to about 200 lbs/min, of the injected material.
Another aspect of the present invention is an apparatus comprising an electric arc furnace having a variable valve in fluid communication with the furnace and a material source that can be controlled to variably adjust the amount of material added to the furnace during a metal processing. Embodiments include an apparatus comprising: an electric arc furnace capable of forming a molten metal covered, in part, by a slag; a sensor in electrical communication with the furnace and capable of sensing the nature or quality of the slag; a material handling and controlling equipment in fluid communication with the furnace for housing and adding material to the furnace to foam the slag and including a variable valve for varying the amount of material added to the furnace; and a programmable logic controller electrically coupled to the sensor and the material handling and controlling equipment for receiving and processing signals from the sensor as to the nature or quality of the slag and for regulating the amount of material added to the furnace by the material handling and controlling equipment through the variable valve.
Additional advantages and other features of the present invention will become readily apparent to those skilled in this art from the following the detailed description, wherein only the preferred embodiment of the present invention is shown and described, simply by way of illustration of the best mode contemplated for carrying out the present invention. As will be realized, the present invention is capable of other and different embodiments, and its several details are capable of modifications in various obvious respects, all without departing from the present invention. Accordingly, the drawings and description should be regarded as illustrative in nature, not as restrictive.